netdev - Re: [PATCH bpf-next 05/13] bpf: get better bpf_prog ksyms based on btf func type

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Message-ID: <20181016175901.vninfgdsjtizssvt@ast-mbp.dhcp.thefacebook.com>
Date:   Tue, 16 Oct 2018 10:59:02 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     ast@...com, kafai@...com, daniel@...earbox.net,
        netdev@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH bpf-next 05/13] bpf: get better bpf_prog ksyms based on
 btf func type_id

On Fri, Oct 12, 2018 at 11:54:42AM -0700, Yonghong Song wrote:
> This patch added interface to load a program with the following
> additional information:
>    . prog_btf_fd
>    . func_info and func_info_len
> where func_info will provides function range and type_id
> corresponding to each function.
> 
> If verifier agrees with function range provided by the user,
> the bpf_prog ksym for each function will use the func name
> provided in the type_id, which is supposed to provide better
> encoding as it is not limited by 16 bytes program name
> limitation and this is better for bpf program which contains
> multiple subprograms.
> 
> The bpf_prog_info interface is also extended to
> return btf_id and jited_func_types, so user spaces can
> print out the function prototype for each jited function.
> 
> Signed-off-by: Yonghong Song <yhs@...com>
...
>  	BUILD_BUG_ON(sizeof("bpf_prog_") +
>  		     sizeof(prog->tag) * 2 +
> @@ -401,6 +403,13 @@ static void bpf_get_prog_name(const struct bpf_prog *prog, char *sym)
>  
>  	sym += snprintf(sym, KSYM_NAME_LEN, "bpf_prog_");
>  	sym  = bin2hex(sym, prog->tag, sizeof(prog->tag));
> +
> +	if (prog->aux->btf) {
> +		func_name = btf_get_name_by_id(prog->aux->btf, prog->aux->type_id);
> +		snprintf(sym, (size_t)(end - sym), "_%s", func_name);
> +		return;

Would it make sense to add a comment here that prog->aux->name is ignored
when full btf name is available? (otherwise the same name will appear twice in ksym)

> +	}
> +
>  	if (prog->aux->name[0])
>  		snprintf(sym, (size_t)(end - sym), "_%s", prog->aux->name);
...
> +static int check_btf_func(struct bpf_prog *prog, struct bpf_verifier_env *env,
> +			  union bpf_attr *attr)
> +{
> +	struct bpf_func_info *data;
> +	int i, nfuncs, ret = 0;
> +
> +	if (!attr->func_info_len)
> +		return 0;
> +
> +	nfuncs = attr->func_info_len / sizeof(struct bpf_func_info);
> +	if (env->subprog_cnt != nfuncs) {
> +		verbose(env, "number of funcs in func_info does not match verifier\n");

'does not match verifier' is hard to make sense of.
How about 'number of funcs in func_info doesn't match number of subprogs' ?

> +		return -EINVAL;
> +	}
> +
> +	data = kvmalloc(attr->func_info_len, GFP_KERNEL | __GFP_NOWARN);
> +	if (!data) {
> +		verbose(env, "no memory to allocate attr func_info\n");

I don't think we ever print such warnings for memory allocations.
imo this can be removed, since enomem is enough.

> +		return -ENOMEM;
> +	}
> +
> +	if (copy_from_user(data, u64_to_user_ptr(attr->func_info),
> +			   attr->func_info_len)) {
> +		verbose(env, "memory copy error for attr func_info\n");

similar thing. kernel never warns about copy_from_user errors.

> +		ret = -EFAULT;
> +		goto cleanup;
> +		}
> +
> +	for (i = 0; i < nfuncs; i++) {
> +		if (env->subprog_info[i].start != data[i].insn_offset) {
> +			verbose(env, "func_info subprog start (%d) does not match verifier (%d)\n",
> +				env->subprog_info[i].start, data[i].insn_offset);

I think printing exact insn offset isn't going to be much help
for regular user to debug it. If this happens, it's likely llvm issue.
How about 'func_info BTF section doesn't match subprog layout in BPF program' ?